1. Field of Invention
The present invention relates to a fuel level system for an automobile, and more particularly, to a fuel level system for use in informing a driver of the amount of remaining fuel in a fuel tank of an automobile using gasoline, light oil, gas or the like, wherein an electric current flowing through a contact of a level sender unit is increased so as to eliminate contact resistance resulting from silver sulfide, silver oxide or other foreign materials generated due to corrosion in the contact of the level sender unit, thereby preventing wrong indication of a fuel gauge due to the contact resistance in the contact and more accurately providing the driver with information on the amount of remaining fuel.
2. Description of the Prior Art
Generally, an automobile is equipped with a fuel level system for informing a driver of the amount of remaining fuel in a fuel tank of the automobile. The fuel level system detects the amount of remaining fuel and then operates a fuel gauge disposed on an instrument panel on the side of a driver's seat.
FIG. 1 is a sectional view schematically showing of a conventional fuel level system for an automobile, and FIG. 2 is a circuit diagram showing one example of the conventional the fuel level system. As shown in the figures, the conventional fuel level system comprises a level sender unit 20 that is installed in a fuel tank 10 and provided with a level resistor 21; a pivotable bar 22 with a movable piece connected to the level resistor 21 of the level sender unit 20 via a contact to change the resistance of the level resistor 21; and a float 23 capable of operating the bar 22 in an interlocked manner.
An ECU 30 receives a voltage drop signal corresponding to the amount of remaining fuel in the fuel tank 10 from the level sender unit 20 and utilizes the signal as an operation factor for checking whether evaporated gas leaks out. The ECU 30 performs the function of converting information on the check results into a pulse width control signal and sends the pulse width control signal to a fuel gauge 40.
Then, the fuel gauge 40 receives the pulse width control signal from the ECU 30 or the voltage drop signal directly from the level sender unit 20, and indicates the amount of remaining fuel in the fuel tank 10 in a graphical manner or using a pointer.
The operational relationship among the above components will be described hereinafter with reference to FIG. 2.
First, the ECU 30 comprises a pull-up resistor 31, and the pull-up resistor 31 and the level resistor 21 of the level sender unit 20 are connected in series via the contact of the movable piece connected to the bar 22. When a 5V constant-voltage power source 32 within the ECU 30 is applied to and energizes the serial circuit, the value of voltage drop (VL) corresponding to the amount of remaining fuel in the fuel tank 10 occurs in the level resistor 21.
Then, an AD converter 33 in the ECU 30 measures the value of the voltage drop through a monitoring resistor 34 and sends a pulse width control signal corresponding to the value of the voltage drop to the fuel gauge 40 so that the amount of remaining fuel can be indicated to a driver.
Meanwhile, as shown in FIG. 3, it is possible to employ a pull-down resistor 35 connected directly to the level resistor 21 instead of the pull-up resistor 31 and to connect the AD converter 33 to the pull-down resistor 35. However, even in this case, the operational relationship is the same.
FIGS. 4 and 5 are circuit diagrams showing other examples of the conventional fuel level system for an automobile. In these examples, the conventional fuel level system only comprises the level sender unit 20 and a microcomputer-type fuel gauge 40 without such an ECU 30. There is a difference in that the microcomputer-type fuel gauge 40 in FIG. 4 contains a microcomputer 41 and a pull-up resistor 42 in FIG. 4, whereas the microcomputer-type fuel gauge 40 in FIG. 5 contains the microcomputer 41 and a pull-down resistor 43.
Additionally, there has been used a fuel level system employing a cross coil-type or bimetal-type gauge as shown in FIG. 6. In this case, the voltage of 12V of a battery in the automobile may be applied to the gauge 50, or a 5˜12V constant-voltage power source 51 may be independently connected to the gauge 50. Alternatively, the ECU 30 may be connected in parallel to use information on determination of the amount of remaining fuel upon control of leak of evaporated gas.
Although various types of fuel level systems have been used as described above, all the fuel level systems utilize in common the contact of the movable piece and the level resistor 21 of the level sender unit 20.
Metals such as AgCu, AgPd or AgNi containing silver (Ag) are mainly used for a conductive portion of the level resistor 21 and the contact of the movable piece. At this time, the silver (Ag) and sulfur (S) contained in fuel react with each other and thus produce silver sulfide as well as silver oxide due to oxidation in the air. Such silver sulfide and silver oxide generate contact resistance in the contact. Thus, there may be a problem in that the fuel gauge of the fuel level system may not indicate a correct value.
As for methods of minimizing such contact resistance, there are the following methods.
First, it is possible to conceive a method by which the contact of the movable piece and the conductive portion of the level resistor are made of a material that cannot be oxidized or corroded. However, there is no feasibility because of a limitation on material technology and high costs.
Second, it is possible to conceive a method by which an oxidized portion can be worn out by increasing contact pressure in the contact of the movable piece during its operation. However, there is a limitation on the durability of the contact.
Finally, it is possible to conceive a method by which an electric current flowing through the contact is increased so that produced contact resistance can be eliminated due to the increased electric current.
To this end, the value of pull-up or down resistance in an electronic control module and the fuel gauge is caused to be decreased and the value of level resistance of the level sender unit is also caused to be decreased. However, in order to cause the fuel gauge to more accurately indicate the amount of remaining fuel, a sufficient gap should be given between the maximum and minimum values of the level resistance. Thus, there is a limitation on the decrease in the value of the resistance.